245.1 - newbie question

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chrise

245.1 - newbie question
« on: 28 Aug 2007, 06:32 pm »
Hi,

I am in the market to move from a EL84 based SET (Decware Zen) to the 45 tube.  I'm very intrigued w/Roger's design, however I'd like to ask any 245.1 users: what's the process one goes through to "dial in" the sound.  My current amp is as simple as plug and play.  This approach is very different (at least for me).  Any other general comments about the amp would be appreciated as well.  It's a real beauty.



Thanks,
Chris Evans   
Denver CO
PS: I'd be running the amp to a pair of Hornshoppe horns (fostex 126e/and "the cube" sub). 


Roger A. Modjeski

Re: 245.1 - newbie question
« Reply #1 on: 30 Aug 2007, 04:13 am »
Chris,

There are two adjustments to make on the amp and I have made them as easy as possible. They are:

1. Adjustable bias control on the rear with LED indicator on top allows any 45 to be set to the correct bias. The pilot LED in the center is your guide for brightness. Just match the adjustment to the center LED and you are done. Takes 15 seconds and only has to be done when changing tubes or if you notice a change in brightness. In your amp there is a cathode resistor which "averages" out the differences between individual EL-84's. Since there is no indicator you won't know where the tube is biased and how much it changes over time. A bit like flying blind. Cathode resistors also tend to rob power.

2. Since the 45 has a directly heated cathode (which is part of it's claim to fame) there is an adjustment to minimize the hum in the speaker. When nulled the amp is very quiet. Again, 15 seconds and stays put till you change the tube. Many listeners claim that AC heaters sound better than DC as long as you can get the hum low enough, which I did. Your amp has an indirectly heated cathode so there is no heater hum to balance out.

One further thing to consider. The EL-84 is a pentode connected as a triode where the 45 is a triode from start. The extra grids in a pentode and the increased spacing between the cathode and plate make it not such a good triode. I like the EL-84 very much and use it in my RM-10 in push-pull pentode operation, but not as a triode.

chrise

Re: 245.1 - newbie question
« Reply #2 on: 30 Aug 2007, 02:16 pm »
Roger,

Excellent description.  Makes perfect sense now.  I will be giving you a call soon.

Much appreciated.

C Evans

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« Last Edit: 30 Aug 2007, 03:42 pm by chrise »